1. Field of the Invention
This invention generally relates to an ultrasonic motor. More particularly, the invention relates to an ultrasonic motor which is used as an actuator.
2. Description of the Related Art
Ultrasonic motors have a simplified structures when compared to the electric motor. Most notably, it eliminates the need for magnets and windings. In general, ultrasonic motors have high torque capabilities even at low rotational velocity. This is due to the ultrasonic vibration actuating system used to drive such motors. Furthermore, high accuracy can be attained in rotational control by designing the system to have friction transmitted between the stator and rotor during operation. These superior features of the ultrasonic motor have lead to their increasing popularity in a wide variety of applications.
One known construction of an ultrasonic motor incorporates a threaded drive shaft. This construction is known for its ability to provide a strong torque. As illustrated in FIG. 3, such a motor may include an annular stator 53 secured to an upper side of a bottom cover plate 52 of a housing 51. A rotor 57 is pressed against the stator 53 by a bevel spring 55 firmly mounted to an upper end of a drive shaft 54 in order to apply a predetermined pressure to the rotor 57.
The stator 53 carries an annular piezo electric element 56. A high frequency voltage is applied to the piezo electric element to cause a circumferentially continuous deformation. Thus, vibrations are generated on a surface of the stator 53 to rotate the rotor 57 without generating any audible sounds. The rotor 57 transmits its rotational movement to the shaft 54 through spring 55.
The shaft 54 is fixed to the rotor 57 at its upper end for integral rotation therewith. A nut 58 is mounted on the shaft 54 and has splined shafts 59 projecting into key ways 60 vertically extending on an inner wall of a case 62. Therefore, the nut 58 engages in the vertically linear movement on the shaft 54 along the key ways 60 in accordance with the rotation of the shaft 54.
As shown in FIG. 4, each spline shaft 59 is detachably connected to a bifurcated tip of a crank shaft 61 of an external body. This arrangement transmits the linear movement of the nut 58 to the external body by way of the crank shaft 61.
In the ultrasonic motor above mentioned, the rotating drive shaft 54 is subject to a thrust load from the nut 58 moving vertically. The thrust road forces the shaft 54 to displace over approximately 500 .mu.m. This results in the change of pressing power of the rotor 55 against stator 53, causing generation of audible sound which is an unacceptable problem in many ultrasonic motor applications.
One proposed solution to the above problem, is to provide a thrust bearing between the housing and the drive shaft. The thrust bearing then receives the thrust load from the shaft. In this motor, however, the thrust bearing must be taken apart anytime the housing is removed from the motor. Therefore, the bearing must be re-assembled whenever the housing is remounted on the motor. This makes the maintenance of the motor very troublesome.
Furthermore, to insure that the thrust bearing works effectively, the housing 51 must be firm and the cover plate 52 must be relatively thick. This adds both weight and size to the ultrasonic motor, which are disadvantageous. It also makes it impractical to manually assemble the motor.
An alternative known ultrasonic motor structure utilizes a drive shaft which carries a gear or the like to transmit power from the rotor to the external body. However, this structure is also susceptible to generating audible sound due to displacement of the drive shaft which is subject to thrust loads caused by general defects in the motor.
The ultrasonic motor disclosed in the Japanese Unexamined Patent Publication 63-73887 seeks to overcome these shortcomings by mounting a radial bearing between the cover plate and the drive shaft. The radial bearing transmits the thrust load from the drive shaft to the cover plate. This motor, however, is not able to convert rotary movement of the drive shaft to the linear movement of the external body. Therefore, there is a need to install a device with a complicated structure for converting the rotational movement of the drive shaft into the linear movement of the external body.